Using cell-free DNA to test for cancer

Loose pieces of DNA course through our veins. As cells in our body die, they cast off fragments of genes, some of which end up in the bloodstream, saliva and urine.

Scientists are excited by the prospect of using cell-free DNA to test for cancer. Instead of relying on invasive biopsies, they hope to find blood-borne fragments that carry distinctive cancer mutations.

Unfortunately, the genetic sequence of a piece of cell-free DNA doesn’t tell researchers where in the body it originated — a valuable clue for doctors looking for diseases. All the cells in our body typically descend from a single fertilised egg, and they inherit all the same genes. The reason we aren’t uniform sacs of protoplasm is that our cells turn those same genes on and off in distinctive patterns, thereby developing into different tissues.

But in a study published in the journal Cell, Jay Shendure, a geneticist at the University of Washington, and his colleagues took some important steps toward identifying the origins of free-floating DNA.

They searched the blood of healthy individuals for cell-free DNA, and then searched a map of the human genome to figure out where each fragment came from. They found that all the healthy subjects produced cell-free DNA that mainly came from nucleosomes found in blood cells. But when they looked at cell-free DNA from people with advanced cancer, the picture was different. In a patient with lung cancer, for example, the team found that the cell-free DNA fit a different pattern — one belonging to a type of lung cancer cell.

The researchers went on to match cell-free DNA in other cancer patients to the types of cancer they had. The findings might eventually make it possible to use cell-free DNA to find important clues about diseases. Doctors might be able to use it to figure out the location of hard-to-find cancers, for example.